1. Field of the Invention
The present invention relates to an inspection for printing (or print data) in a printing work flow, and more particularly to an inspection of digital print data.
2. Description of the Background Art
Inspection for printing generally refers to an operation which is performed before generation of a plate-making film and a printing plate in a printing work flow, to check if there are any error in a camera-ready art, a film or the like which are respective original bases of the printing plate, the plate-making film, or the like. Recently, with progress of DTP (desktop publishing) technique, an apparatus has been well-known which performs an inspection of digital print data, together with color proof, by comparing print data of the first proof and that of the revised proof with each other, both of which are processed (rasterized) by an RIP (raster image processor), by pixel on a display screen to detect a differential value of color density (tone value) of each pixel. Since a CTP (Computer To Plate) process for direct plate making from the print data which is processed by the RIP and a digital printing for directly producing a printed matter have become general work flows in these days, the inspection of digital print data becomes increasingly important.
Among such conventional processing methods for inspection of digital print data is a so-called “swing (shift) method”. This is a method, as disclosed in, e.g., Japanese Patent Application Laid Open Gazette No. 9-166864, where even when an arrangement position of a linework image or a picture image in image data (object image data) of a print image (object image) to be inspected deviates from that in image data (reference image data) of a print image (reference image) serving as a reference for inspection, and in other words, there is a pixel displacement, the pixel displacement can be cancelled to allow detection of a proper differential value or the like by virtually shifting (moving in parallel) the arrangement position of either image data and then comparing these image data with each other.
When the pixel displacement is caused by half pixel (in subpixel order), not by pixel, in the linework image and the picture image, however, the above swing (shift) method arises a problem of not correctly canceling the displacement. In a case of linework represented by binary data, for example, when there is a pixel displacement by half pixel between the arrangement position of the reference image and that of the object image before RIP development, there arises a difference in jaggies between both image data generated as the result of the RIP development. It means that there is a difference in form of linework between the reference image data and the object image data. In a case of multitone picture image, when there is a pixel displacement by half pixel, an unnecessary interpolation is made within a range of the pixel positions where the displacement occurs and pixels of intermediate value are thereby generated. The same phenomenon occurs also when scaling of an image is performed after the RIP development. There is a processing case for permitting such a half-pixel displacement or the like, for example, where it is assumed that there is no pixel displacement if a differential value is equal to or lower than a predetermined threshold value, or the like. But such a processing is not practical since when a pixel of intermediate value is generated between a pixel of blank fill having a tone value of 0% and a pixel of solid fill having a tone value of 100%, it is necessary to provide a tolerance of 50% in tone value in order to permit this displacement.
On the other hand, while the conventional inspection for printing on the basis of a printed matter which is actually outputted is performed by visual check and therefore its detectivity is limited within the image resolution of human's visual property, the inspection of digital print data arises a new problem that an invisible difference is detected since comparison is made by using numeric data such as a tone value of image. In many cases, the invisible differences do not need to be detected but are detected in a large number. This causes a disadvantageous situation where a “difference to be properly detected” is mixed into information on the unnecessarily-detected differences and eventually ignored.
When a resolution of image data to be inspected is 400 dpi, for example, if the displacement occurs within several pixels, this displacement does not become a problem in most cases since it can not be detected by the visual check. As a matter of course, a displacement within less than one pixel (i.e., subpixel) does not become a problem.